I use twitter a lot.
Opera is a useful browser because it is super super fast. Fast as both in downloading (it compressed data thru its man server) and fast in operation.
Any one of the flash light utilities is helpful.

I think an easy way for you to discover is to go the the market, click on Apps, -> All Apps -> top-free and start there.

More useful apps:
-Antennas
-Gmote (haven't tried it out yet, but pumped to set it up)
-imdb came out recently.
-GroceryIQ has "awesome" potential but I haven't used it yet.
-listen for podcasts (by Google)
-My Tracks for tracking running, biking, etc.
-scoreboard (by Google)
-speed test
-system panel ($3 or so but good for trouble shooting issues with battery, charging, etc)
-WiFi Analyzer
-UPS mobile is ok

No problem. Now I need to convince my wife that we should get HTC Arias. I think she's mostly ambivalent about them due to the extra $30-50/mo for data plans since we don't have smartphones now. Anyone care to share what apps, uses, etc particularly appeal to women? I've read about these phones so much lately I don't even know where to begin... I feel like I know what they can do but it's from a bunch of different sources, and I need to distill it down because she doesn't have the time or interest to look into it herself. Perhaps if I tell her that I can read the FAP materials anytime I have a few minutes to kill...

For the record, what sold her on getting smart phones was the ability to instantly look something up online to prove to me when I am wrong